This invention relates to a communications network with optical channels which are provided in large numbers following the heterodyne principle.
In such a communications network, the transmission medium, an optical fiber, is operated in a light carrier frequency multiplex technique. A single common optical fiber thus offers the subscribers connected to it a very large number of channels, on which they can have access which is optional in the case of distribution services, for example radio programs, or by using a switching control system in the case of exchange services, for example telephone, video telephone and data communication.
The invention thus differs from previously known systems of optical communications technology in a manner which can be illustrated by comparison with the development of radio technology. Initially, the so-called direct reception was achieved here and there. In this case, transmission and reception are carried out on a particular wavelength or, in some cases, on some few particular wavelengths. The use of the superposition principle, which makes possible a substantially increased threshold sensitivity and selectivity compared with such arrangements, does, however, make it possible to utilize the transmission capacity of the medium completely. The advantages which this principle brought to radio technology are of outstanding importance but their importance has been generally forgotten in the meantime. The "Superhet" receiver helped radio technology to its first real breakthrough and is now standard in that field.
The heterodyne principle is therefore also of great interest to optical communications systems. Several publications, for example in "Electronics Letters" volume 16 (1980) pages 179 to 181, pages 709/710 and pages 826/827, refer to special details of the system components required, particularly the thermal stabilization of semiconductor lasers and corresponding control circuits. In the IEEE Journal of Quantum Electronics, volume QE-17 number 6, June 1981, pages 919 to 935, there is a comprehensive survey in which the properties and problems of the systems and system components of a coherent optical fiber transmission system are discussed in great detail. In the Conference Proceedings pages 7.1-1 to 7.1-3 of the 7th European Conference on Optical Communication, Copenhagen, Sept. 8-11th 1981, a point to point connection for a future optical heterodyne system with monomode fibers as transmission medium is shown as a block circuit diagram and described together with its components, particularly those involved in transmission and reception.
The main objective apparent from these publications is to achieve a distance between repeaters or a transmission length of approximately 100 km to 200 km, so that submarine connections can be realized in this manner. In this connection, it is assumed that there will be one fixed frequency channel for each pair of communications source and sink.